The gap between Americans who are church members and nonmembers who hold spiritual values appears to be widening. This trend runs through several new surveys on religion and its place in public and private life.

For example, the National Council of Churches' (NCC) ``Yearbook of American and Canadian Churches, 1988,'' which is being released today, reports that the number of church members in the US remained virtually unchanged from 1985 to 1986, while the general population grew at a slower rate during the same period.

``The statistics do not show any significant growth in the religious sector,'' says Yearbook editor Constant Jacquet.

``Mainline losses continue, but are moderating,'' he explains. ``At the same time, the trend toward gains in some conservative churches is also moderating.''

Mr. Jacquet stresses, however, that ``many denominations show an increase of per capita giving well above the rate of inflation. This is also a measure of religiosity.''

The NCC study shows that US religious bodies showing modest membership increases include the Assemblies of God, the Christian and Missionary Alliance, Jehovah's Witnesses, the Presbyterian Church in America, and Seventh-day Adventists.

A Gallup poll reported in July stressed a continuing lag between commitment to things of the spirit and church attendance.

``The churches of America have made no headway in narrowing the gap between religious belief and church involvement, between believers and belongers,'' says George Gallup Jr., who heads the Princeton Religious Research Center.

This nationwide survey is a follow-up to a similar Gallup poll in 1978. It centers on the ``unchurched American,'' indicating that 44 percent of respondents were classified ``unchurched'' because they have not attended regular services in six months. Just 40 percent of those polled a decade ago were in this category.

According to Gallup, 61 million US adults did not belong to any church in 1978. This number has increased to 78 million today. This analysis stresses, however, that being ``unchurched'' does not necessarily indicate a lack of faith.

The poll showed that 72 percent of those who do not attend church believe in Christ, 77 percent believe in God, 63 percent believe the Bible is ``inspired,'' and 25 percent say they have had a ``religious experience.''

Dr. Gallup says that ``the churches have done well to keep slippage at a minimum in view of the continued high mobility among Americans during the last decade, the distractions of modern life, and the apparent growing appeal of cults and nontraditional religious movements.''

A rosier picture of religious commitment surfaced in a Better Homes and Gardens survey of its readers conducted earlier this year.

The magazine interviewed Roman Catholics, Protestants, and Jews, as well as those of other religions and no religion. It found that 96 percent believed in God; 50 percent said that spirituality is gaining influence on family life in the US, and 79 percent indicated that they attended a church, temple, or synagogue regularly.

Some interpreters of religious life in the US say the media have placed the wrong emphasis on church trends.

James Castelli, whose new book analyzes the clash between religion and politics, has said: ``Twenty-five years ago, the media focused primarily on the National Council of Churches, which did not represent the dominant religious view of the country. That was poor coverage. They are doing the same thing today, only they're focusing on a different group, the evangelicals, who are obviously important as are the mainline churches. But they are not the dominant religious force in the country either.''

Meanwhile, church and state issues - including school prayer and government aid to religious institutions - continue to surface in public debate.

But many say they are not as important issues to many Americans as they were a decade ago, or even during the last presidential election.

Of late, a nationwide controversy has arisen over the motion picture, ``The Last Temptation of Christ.''

Most religious groups have condemned the film for distorting the life of Jesus. Some individuals and groups have demanded that it be banned, protesting in front of theaters where it is shown.

In response, A. James Rudin, director of interreligious affairs for the American Jewish Committee, now calls for religious leaders and representatives of the film industry to join in a conference aimed at defusing the tensions aroused by this movie.

Rabbi Rudin says that such a meeting ``would not be intended to stifle creative talents, nor to silence the valid concerns of the various religious groups, but to break down the harmful stereotypes and caricatures that have surfaced during this controversy.''

Believing and belonging Percent of Americans not attending church on a regular basis who:

1978 1988 Believe Jesus Christ to be God or the Son of God 78% 84% Ever pray to God 76% 77% Have made a ``commitment to Jesus Christ'' 60% 66% Believe the Bible is the inspired word of God * 78% Have had a religious experience 24% 25% *Survey question was different in 1978 SOURCE: PRINCETON RELIGIOUS RESEARCH CENTER, GALLUP POLL